Genetic approaches are being used to study the regulation of the functions of T4 phage proteins that play roles in viral DNA replication. In some of our studies, we are attempting to correct the effects of genetic defects in certain proteins by altering other proteins with which they interact in the infected cells. In other studies, we are isolating mutations that alter the timing and amount of synthesis of DNA replication proteins. Such mutations are useful in two regards: (a) some result in hyperproduction of proteins and therefore facilitate the isolation of these proteins for in vitro studies of replication,and (b) some can be used to study the possible need for accurate assembly of replication enzyme complexes for viability of the virus.